This invention relates to heating thermoplastic preforms to molding temperature and more particularly to heat programming improvements applicable to such heating.
It is known to heat thermoplastic preforms to molding temperature prior to distension (for example axially and circumferentially) in a mold cavity into hollow article configurations. With some thermoplastics it is necessary to control the temperature of the material quite precisely since melt strength varies appreciably over a relatively narrow temperature range, especially around the molecular orientation temperature of the material. To accomplish such control and affect material distribution in the finished article, selective programming of heat into particular portions of the preforms has been practiced. Thus, systems are known wherein banks of vertically arranged resistance heaters are used to bring previously formed preforms to molecular orientation temperature. When such systems were used with certain thermoplastics sensitive to heat, for example the high nitrile materials, a phenomenon known as "banding" occurred in that optically unattractive thickness variations around circumferentially extending portions of the finished article were apparent to the naked eye. This was believed caused by uneven heating of the preform by individual heaters in that input to a particular short vertical zone of influence of the preform from an opposite heater tended to be greater toward the middle of such zone than toward its ends with the result that the middle during molding stretched more than the ends to produce the uneven wall and the visual banding effect.
To improve control and avoid such heat concentration, in one case individual heaters of an opposing pair were staggered or offset vertically from one another to a horizontally non-aligned disposition whereas in another case such heaters were tilted along their length from the horizontal in the direction of advancing preform movement. However, each of these approaches detracted from the ability to closely control the evenness of heating in each vertical zone of the preform by a particular heater or heater pair in that the effect of adjacent heaters preset at different temperatures overlapped opposing adjacent zones of the preform, when in fact it is desired that each preform zone be influenced by a single heater or opposing heater pair.